HTML search: adjust type signatures for custom HTML scoring

[jayaddison]

Feature or Bugfix

• Refactoring

Purpose

• Adjust the type hint for the scoring argument to the IndexBuilder

Detail

• Currently we have test coverage that provides a scoring value of None -- but the type signature we have on the class doesn't accept that, strictly speaking.
• Adjust the type signature to accept that single additional value (since we already have test coverage for it).
• Also: add a str type to the relevant HTML builder config value.

Relates

• Originates from discussion in Further improve typing in tests/ #12816.

cc @danieleades

[AA-Turner]

I think '' is fine (& documented as the default)